This invention generally relates to pumpdown toolstring operations in horizontal or high-deviation oil or gas wells, including operations requiring application of power to operate downhole tools in such wells.
Methods and systems for maintenance and service operations in wells through flow-lines are known in the art. Thus, it is known to employ a toolstring adapted to be pumped down in tubing means with a circulation path including, for example, the annulus between the tubing and a casing in the well. A toolstring of the type contemplated here may be quite long, such as several hundred meters so as to be able to perform operations along the length of the horizontal or highdeviation part of a well, which may extend for as much as 1000 meters from a vertical part or more specifically from a bend part of the well. Known toolstring designs comprise so-called locomotives or piston-like drive means for the pumping action downwards and upwards in the well respectively.
Examples of methods and equipment for performing operations in horizontal or highly deviated wells can be found in U.S. Pat. Nos. 4,729,429, 4,484,628, 4,125,162, 4,062,403, 4,027,730 and 3,957,119. U.S. Pat. No. 4,729,429 involves logging or other measurements in a horizontal or highly deviated production or injection well. According to this patent specification, a wireline is necessary for the intended function. Transport of the wireline along the highly deviated well part or section is achieved by pushing a long tubing element with a locomotive at its top end. Apparently, there will be a problem of buckling of such a long tubing element. Moreover, the fluid displaced below the locomotive must be injected into the surrounding formation. The toolstring is retrieved by pulling on the wireline. It is also to be noted that according to this prior patent operations are performed under wellhead pressure.
U.S. Pat. No. 4,484,628 relates to logging in open or cased wells which may be horizontal or highly deviated. A workstring is used to transport the logging tool to the top of the section to be logged. An electric line is then lowered and pumped down to the bottom of a tubular extension to the tool. This extension is thereafter scoped out of the end of the workstring in order to transport the tool further to the bottom of the section, which is then logged by pulling the tool and extension back into the workstring with the electric line.
The remarks immediately above also apply to a further a U.S. Patent publication, namely U.S. patent reissue No. 32336.
U.S. Pat. No. 4,027,730 describe pumpdown services involving circulation through a length of so-called coiled tubing. This patent specification is particularly directed to means of improving the circulating capabilities of various toolstrings.
U.S. Pat. No. 4,062,403 is somewhat more interesting in connection with the present invention than the patent specifications discussed above. U.S. Pat. No. 4,062,403 also relates to pumpdown services involving circulation through a length of coiled tubing, and is specifically directed to sand washing. Although not mentioned in this patent specification, the techniques described are suitable for horizontal or highly deviated wells, provided that the horizontal length to be traversed is not so long as to induce buckling of the coiled tubing. However, since sand washing is a dominating consideration, a dual tubing string is essential. Such a dual tubing constitutes a severe production restriction for prolific wells. Although the present invention in one embodiment may be employed in connection with dual tubing strings, it is not restricted to such arrangement, and is clearly considered to be of more importance in arrangements with a single production string or tubing and incorporating the well annulus in a complete circulation path. Also of interest is U.S. Pat. No. 3,957,119, showing a type of piston or locomotive for use in pulling a toolstring into tubing below a circulation point, this operation being dependent on the existence of an exit point for power fluid lower in the well. For a typical application, hydrostatic pressure must then be balanced to formation pressure in order to avoid losses to the formation or influx of formation fluids. This is extremely difficult under realistic field conditions, for example due to inaccurate knowledge of reservoir pressure.
U.S. Pat. No. 4,441,558 describes a kind of operation being a typical example of operations with which the present invention is concerned. The same applies to U.S. Pat. No. 3,051,243.